High-pressure discharge lamps usually have a fill of a noble gas and, if desired, mercury and/or a metal halide therein. High-pressure discharge lamps which operate under very high pressures, and particularly higher powered high-pressure discharge lamps, have arc tubes which include a discharge vessel from which hollow necks extend into which the electrode shafts are melted-in. The long, extending necks permit placement of sealing foils for electrical and sealed connection of the electrode shafts and their current supplies as far from the arc as possible, in order to avoid problems arising in connections with different thermal coefficients of expansion of the respective materials. Typically, the discharge vessel is made of quartz glass. Sealing problems arise if heat from the discharge arc is transferred to the connecting foils, usually of molybdenum. Quartz glass from the extending neck of the discharge vessel must not touch the electrode shaft in the region which faces the discharge arc and is located between the sealing foil and the melt-in end of the electrode connection. The melting-on of the neck to the quartz vessel may be done on a machine. If quartz glass adheres in the melt region, and is not immediately removed, differential thermal coefficients of expansion of the attached quartz glass and of the electrode shaft, typically of tungsten, will cause fissures and cracks to appear which, as the lamp is used, will lead to failure of the lamp when the electrode, as it expands under heating, cracks the quartz glass in the region of the electrode shafts. In order to prevent such cracking, it has been customary to seal in the necks and electrodes of such lamps by hand, rather than by automatic machinery. This, of course, substantially raises the cost of such lamps.
U.S. Pat. No. 3,742,283, Loughridge, discloses an arrangement in which the electrode shafts, in the region of a pinch seal, are surrounded by concentric tubes made of Cermet, in order to reduce stresses in the region of the pinch seal, and to hold any thermal stresses to a reasonable level. Cermet is a melt compound made of powdered metal and quartz glass. These Cermet tubes have thermal coefficients of expansion which are intermediate that of the quartz glass and of the metallic electrode shaft.